This invention relates to a method for heat treatment of fine material using a flash calciner. The method specifically relates to phosphate calcination, as well as calcination of various ores. The present method can also be utilized to process fines generated from activated carbon rotary kiln systems.
Pyrometallurgical operations when applied to ores generally alter the chemical and physical properties of the materials processed. These known processes are characterized by chemistry which mainly involves gas-solid reactions such as calcination.
A basic pyrometallurgical operation is the decomposition of hydrates and carbonates. A typical reaction is the decomposition of pure limestone (CaCO.sub.3) to calcium oxide and carbon dioxide: EQU CaCO.sub.3 (s).fwdarw.CaO(s)+CO.sub.2 (g)
where the symbols (s) and (g) represent the solid and gaseous state, respectively. This reaction is strongly endothermic, and in a conventional processing system requires high heat input and long retention times.
Lime is an important raw material for the metallurgical industry. It is used primarily as a flux in smelting and converting, but it is also a neutralizing agent for hydrometallurgical processes. The calcination of magnesite (MgCO.sub.3) yields magnesia (MgO) which is an essential raw material for furnace refractories. The calcination of dolomite (CaCO.sub.3 .multidot.MgCO.sub.3) also yields a calcine used as a feed for the preparation of magnesium metal, as well as other uses.
Other examples of similar process are phosphate calcination and gold ore processing.
The unique characteristics of flash calcination are particularly suited to processing phosphate. Phosphate is a complicated mineral, that varies from deposit to deposit with each ore requiring its own special processing considerations. When phosphate rock is processed to produce super-phosphates and phosphoric acid, thermal processing is required to remove carbon, sulfides and certain trace elements. In addition, there can also be some upgrading of the P.sub.2 O.sub.5 content by the elimination of certain components such as lime. In processing, it's important not to destroy the delicate crystal structure of the phosphate by overheating. This is important because the porosity of the phosphate is very important in the digestion steps that follow calcination. Flash calcination offers the unique advantages of very short retention times, high heat transfer rates, very good oxygen contact and rapid cooling. All of these characteristics are very important in the production of high quality calcined phosphate.
Many gold ores contain sulfides and carbon, which result in high cyanide consumption in the recovery process. Heat treating the ore prior to the cyanide leach will reduce cyanide consumption, which is vital to the economics of processing. Processing in an oxidizing atmosphere where there is above average mixing of the ore with the gas is very important to the oxidizing roast. For the present invention, a properly sized ore when processed in the flash calciner is subjected to excellent dispersion and mixing in the gas stream. Oxygen and temperature can be controlled over the whole length of the calcining zone.
Applicants method achieves energy conservation by control of temperature and gas atmosphere of a flash calcination unit during a protracted residence time of the material being calcined.
The system of the present invention offers several advantages compared to the prior art. The system permits maximum gas to solid contact for reaction to take place. Excellent control is permitted. The present system also has high fuel efficiency as a result of heat recovery from solids and gases. Excellent temperature control is achieved for heat sensitive products and flame contact can be eliminated if desired. The present system can also eliminate reduction gas components internally ahead of the preheat system immediately after the reaction furnace using bleed air which eliminates the use of an external afterburner. With these advantages in mind, applicants will begin to describe in greater detail their discovery and achievements.